1. Field of the Invention
The present invention relates generally to a constant voltage circuit that uses an error amplifier, and particularly to a technique for increasing the response speed for responding to an abrupt change in an input voltage or a load current.
The present invention also relates to a constant voltage circuit that uses an error amplifier including a frequency compensation circuit that conducts phase compensation.
2. Description of the Related Art
Conventionally, an error amplifier used in a constant voltage circuit includes a frequency compensation circuit that conducts phase compensation in order to prevent unstable operation such as oscillation.
FIG. 1 is a circuit diagram showing an exemplary configuration of a constant voltage circuit according to the prior art.
In the constant voltage circuit 100 of FIG. 1, an error amplifier AMPa includes NMOS transistors M103 and M104 that realize a differential pair, PMOS transistors M105 and M106 forming a current mirror circuit that realizes a load of the differential pair, and a NMOS transistor M102 that corresponds to a constant current source that supplies a bias current to the differential pair. Further, the error amplifier AMPa includes a PMOS transistor M107 and an NMOS transistor M108 that realize an output circuit, and a resistor R103 and a condenser C101 that realize a frequency compensation circuit.
In the error amplifier AMPa as is described above, a divided voltage VFBa of an output voltage Vout that is generated by resistors R101 and R102 is input to a gate of the NMOS transistor M104 corresponding to a non-inverting input terminal, and a predetermined reference voltage Vs from a reference voltage generating circuit 101 is input to a gate of the NMOS transistor 103 corresponding to an inverting input terminal. The error amplifier AMPa conducts operation control of an output voltage control transistor M101 so that the divided voltage VFBa corresponds to the reference voltage Vs, and controls the current that is output from the output voltage control transistor M101 to a load.
It is noted that various applications of a constant voltage circuit have been developed in the prior art. For example, Japanese Laid-Open Patent Publication No. 2001-101862 discloses a semiconductor device that is capable of stabilizing an output voltage of a power source circuit and reducing current consumption at the same time. Japanese Laid-Open Patent Publication No. 2002-312043 discloses a voltage regulator that is capable of increasing a response speed according to a load state, and reducing the current consumption rate without increasing the chip area.
Also, Japanese Laid-Open Patent Publication No. 11-150428 discloses a differential amplifier that is capable of easing the gain decrease at a high frequency band of an input signal.
Generally, an error amplifier of a constant voltage circuit is designed to have good direct current characteristics. In this regard, the error amplifier is arranged to realize a high direct current gain, and in turn, the bias current supplied to the differential pair is arranged to be low. However, in such an arrangement, a relatively long period of time is needed to charge the condenser C101 of the frequency compensation circuit and the input capacitance of the output voltage control transistor M101, and consequently, the response speed for responding to an abrupt change in an input voltage Vin or a load current may be relatively slow.